Source: HackerRank > Data Structures > Arrays > Arrays - DS

Description

Print elements of an array in reverse order

Given an array of numeric, character, or string values, print each element in reverse order as a single line of space-separated elements.

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[1, 4, 3, 2]         ⟹ "2 3 4 1"
["a", "b", "c", "d"] ⟹ "d c b a"

My solution

Using reverse

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#lang racket

(define (reverse-str lst)
 (string-join (map ~a (reverse lst))))

Without using reverse

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#lang racket

(define (reverse-str lst)
  "Given a list, return a string that reverses the list."
  (local [
    (define (all-but-last lst)
      "Return all but the last element of a list."
      (take lst (- (length lst) 1)))
    (define (stringify elt)
      "Convert an element to a string."
      (if (number? elt)
        (number->string elt)
        elt))
    (define (reverse-str lst str)
      (if (= 1 (length lst))
          (string-append (stringify (last lst)) str)
          (string-append
            (stringify (last lst))
            " "
            (reverse-str (all-but-last lst) str))))]
    (reverse-str lst "")))


(define (print-reverse lst)
  (print (reverse-str lst)))


(provide reverse-str)

Tests

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#lang racket

(require rackunit rackunit/text-ui "print-reverse.rkt")

(define print-reverse-tests
  (test-suite "print-reverse-tests" 
    (test-case "Should return a string with numeric list elements reversed"
        (check-equal? (reverse-str '(1 4 3 2)) "2 3 4 1"))
    (test-case "Should return a string with string list elements reversed"
        (check-equal? (reverse-str '("a" "b" "c" "d")) "d c b a"))
    ))

(run-tests print-reverse-tests)